The present invention provides a series of process and apparatus modifications to ultrasonic holography imaging systems that each or together function to enhance image quality of ultrasonic holography images. Specifically, the present invention provides a process and apparatus for generating multiple exposure ultrasonic holography images generated from selected orientations, each of which can provide multiple images, utilizing multiple intensities, and multiple frequencies from each orientation. The inventive process and apparatus for providing multiple view, multiple angle, and multiple frequency or intensity transmissive ultrasound imaging of the internal structures of an object provides an object sound (ultrasound or ultrasonic energy) intensity of equal or near equal intensity across the entire field of the object, such as a human breast.
A central element field of holography is fulfilled by combining or interfering an object wave or ultrasonic energy with a reference wave or ultrasonic energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial source of the object wave and reference wave or energy are coherent with respect to the other wave. All parts of both the object wave and the reference wave are of the same frequency and of a defined orientation (a fixed spatial position and angle between the direction of propagation of the two sources). When performing holography the object wave is modified by interference with structure within the object of interest. As this object wave interacts with points of the object the three-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern which identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstructing source in a manner to represent the 3-D nature of the object, as seen by the ultrasonic object wave.
To reiterate, to perform holography coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies), as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range. In the practice of ultrasound holography, one key element is the source of the ultrasound, such as a large area ultrasound transducer. A second key element is the projection of the object wave from a volume within the object (the ultrasonic lens projection system) and a third is the detector and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence as compared to another wave of equal characteristics. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude reduces or eliminates the xe2x80x9cedge effectxe2x80x9d from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through transmission ultrasonic holographic imaging, an ultrasonic energy pulse from the object transducer progresses through the object, then through an acoustic focusing lens and at the appropriate time, a second pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create a interference pattern (the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central imaging axis of the lens means. This is provided by either positioning or rotating the object transducer around the central axis of the lens means or by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of the lens means.
With a through-transmission imaging system, it is important to determine the amount of resolution in the xe2x80x9czxe2x80x9d dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit of one half the wavelength of the ultrasound used. However, it may be desirable to limit the xe2x80x9czxe2x80x9d direction image volume so that one can xe2x80x9cfocusxe2x80x9d in on one thin volume slice. Otherwise, the amount of information may be too great. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens means that will allow the imaging system to xe2x80x9cfocusxe2x80x9d on a plane within the object. Additionally, this lens system and the corresponding motorized, computer controlled lens drive will allow one to adjust the focal plane and at any given plane to be able to magnify or demagnify at that z dimension position.
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface and reconstructing the image by reflecting light from the surface. However, there is no equivalent xe2x80x9cfilmxe2x80x9d material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot propagate through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short (xcex(wavelength)=v/(velocity)/f(frequency)). The density of air (approximately 0.00116 g/cm3) is not sufficient to couple these short wavelengths and allow them to propagate. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such sound. For example, the velocity of sound in air is approximately 330 meters/second whereas in water it is approximately 1497 meter/second. Thus, for water, both the density (1 g/cm3) and the wavelength (xcx9c1.48 mm at 1 MHz) are significantly large such that ultrasound can propagate with little attenuation. Whereas, for air both the density (0.00116 g/cm3) and wavelength (0.33 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Since ultrasound (or sound) propagates as a mechanical force it is apparent that the reflection (or changing direction of propagation) will impart a forward force on this liquid air interface. This force, in turn, will distort the surface of the liquid. The amount of surface distortion will depend upon the amplitude of the ultrasound wave at each point being reflected and the surface tension of the liquid. Thus, the pattern of the deformation is the pattern of the phase and amplitude of the ultrasonic wave.
It is in this manner that a liquid-air interface can be commonly used to provide a near real-time recorder (xe2x80x9cfilm equivalentxe2x80x9d) for an ultrasonic hologram. The shape of the surface deformation on this liquid-air detector is the representation of the phase and amplitude of the ultrasonic hologram formed by the interference of the object and reference ultrasonic waves.
The greatest value of the ultrasonic holographic process is achieved by reconstructing the hologram in a usable manner, usually in light, to make visible the structural nature of the initial object. In the case of a liquid-air interface, the reconstruction to achieve the visible image is accomplished by reflecting a coherent light from this liquid-air surface. This is the equivalent process to reflecting laser light from optically generated hologram that is embossed on the surface of a reflecting material (e.g., thin aluminum film).
The reflected light is diffracted (scattered) by the hologram to diffracted orders, each of which contains image information about the object. These diffracted orders are referred to as xc2x1n th orders. That part of the reconstructing light that does not interact with the hologram is referred to as zero order and is usually blocked so that the weaker diffracted orders can be imaged. The higher the diffracted order the greater the separation angle from the zero order of reflected light.
Once reconstructed, the image may be viewed directly, by means of a video camera or through post processing.
Ultrasonic holography as typically practiced is illustrated in FIG. 1. A plane wave of sound 1a (ultrasound) is generated by the transducer 1 (U.S. Pat. No. 5,329,202 incorporated herein by reference). The sound is scattered (diffracted) by structural points within the object within the focal plane 2. This sound 2a is scattered from the internal object points that lie in the focal plane 15 are focused (projected) into the ultrasonic hologram plane 6. The focusing takes place by use of ultrasonic lens 3 (U.S. Pat. No. 5,235,553 incorporated herein by reference) which focuses the scattered sound into a hologram detector surface 6 and the unscattered sound into a focal point 4. The lens means also allows the imaging process to magnify the image or change focus position (U.S. Pat. No. 5,212,571 incorporated herein by reference). Since the focus point of the unscattered sound 4 is prior to the holographic detector plane 6, this portion of the total sound again expands to form the image from the transparent image contribution (that portion of the sound that transmitted through the object as if it were transparent or semitransparent). In such an application, an ultrasound reflector 5 is generally used to direct the object sound at a different angle (preferably vertically to allow for the holographic detector to have a surface parallel to ground to avoid gravity effects), thus impinging on horizontal detector plane usually containing a liquid which is deformed by the ultrasound reflecting from the liquid-air interface. When the reference wave 8 and the object wave are simultaneous reflected from this detector, the deformation of the liquid-air interface is the exact pattern of the ultrasonic hologram formed by the object wave 1a combined with 2a and the xe2x80x9coff-axisxe2x80x9d reference wave 8.
This ultrasonic hologram formed in the holographic detector 6 is subsequently reconstructed for viewing by using a coherent light source 9, which may be passed through an optical lens 8, and reflected from the holographic detector surface (U.S. Pat. No. 5,179,455 incorporated herein by reference). This reflected coherent light contains two components. These are A: The light that is reflected from the ultrasound hologram which was not diffracted by the ultrasonic holographic pattern which is focused at position 13 and referred to as undiffracted or zero order light; and B: The light that does get diffracted from/by the ultrasonic hologram is reflected at an xe2x80x9coff-axisxe2x80x9d angle from the zero order at position 11 and referred to as the xe2x80x9cfirst orderxe2x80x9d image view when passed through a spatial filter 12. It is noted that this reconstruction method produces multiple diffraction orders each containing the ultrasonic object information. Note also both + and xe2x88x92 multiple orders of the diffracted image are present and can be used individually or in combinations to view the optical reconstructed image from the ultrasonically formed hologram by modifying the spatial filter 12 accordingly.
Commercial application of ultrasonic holography has been actively pursued over many years, yet only limited results have been achieved. The application of ultrasonic holography has commercial utility for non-destructive testing of materials and imaging of internal structures in soft tissue. One of the problems often encountered is consistency and quality of images obtained. It is difficult to obtain undistorted images of selected internal structures within objects (such as a human breast) due to interference or shadowing of other out-of-focus structures within the object.
Therefore, there is a need in the art to improve image quality by recognizing and utilizing the effects of diffraction generated by internal structures within the object. This need is particularly strong for breast cancer screening techniques that now utilize invasive mammography (providing the patient with a dose of radiation from X-Ray imaging) and yet do not produce images that are sensitive to detecting some lesions and do not lend a sense of three dimensional structure to breast tissue.
That portion of the ultrasound wave that passes through the imaged object without being scattered by structures within the object can be a major contributor in xe2x80x9csemitransparent objectsxe2x80x9d (that is, an object that scatters a small portion of the sound waves directed at the object). Since many objects of interest can be rather transparent to sound, (e.g. human soft tissue normal structures and tumor tissue of solid tumors) there is formed a bright and strong white light contribution to the image from this sound that does not interfere with the object. When one wants to detect and determine the characteristic of subtle changes in the object (e.g., determining tissue characteristics) this background bright image contribution can overpower the resolution of small and subtle contributions of tissue change. Therefore, there is a need in the art to improve resolution characteristics of transmissive ultrasonic imaging so as to be able to distinguish subtle differences within the object (i.e., so as to be able to image tumor tissue within surrounding soft breast tissue).
In U.S. Pat. No. 5,329,817, an ultrasonic holography imaging process and apparatus embodiment is disclosed that provides for a rotating single ultrasonic transducer (FIGS. 7-9) along with an angled rectangular transducer at an angle xcex8 with respect to the normal plane or axis of the xe2x80x9csystemxe2x80x9d (e.g., centerline of the acoustic lens means). The single ultrasonic transducer element is angled (xcex8) at an acute incidence angle relative to the optical axis to better remove imaging shadows from out-of-focus (i.e., the focal plane of the object) internal structures of the object.
The present invention provides an ultrasonic holography imaging apparatus comprising:
one or a plurality of ultrasonic transducers directing ultrasonic energy in the form of a wave toward an object to be imaged;
an acoustic lens for focusing the ultrasonic energy to a focal point downstream of a first lens and having a centerline; and
a holographic detector having a surface aligned perpendicular to the centerline of the acoustic lens means.
The present invention provides a process for generating an image using an ultrasonic imaging apparatus, comprising the steps of:
providing an object to be internally imaged to be held by the object holder;
transmitting a sequence of individual pulses of ultrasound, each pulse within the sequence comprises: a plurality of cycles of a single frequency (f) of ultrasound, wherein each pulse has a multitude of characteristic parameters; each sequence is composed of a plurality of pulses; and one or more the characteristic parameters is varied from one pulse to another pulse within the same sequence; and
imaging the object from a hologram formed in the holographic detection system for each pulse within the sequence.
Preferably, the process further comprises either capturing each separate image for separate analysis for a specific frequency, or averaging a plurality of images from selected frequencies to form a composite image derived from the selected frequencies.
There are a number of characteristic parameters for a pulse of ultrasound used with respect to the present invention. One parameter is the frequency of the pulse, namely, the frequency of the acoustic wave. A second parameter is the magnitude of the acoustic wave, also termed the amplitude, as represented by the peak-to-peak value of the sound wave. Another characteristic is the angle at which the sound wave is directed towards the object under study. There may be other characteristic parameters which may also be varied according to principles of the present invention; the three being provided are examples of suitable parameters to achieved an improved holographic image as explained in more detail herein.
In one embodiment, the present invention provides an improvement to the device and process of ultrasonic holography imaging, especially in imaging for tumor masses in soft tissue. Each incremental improvement to either the apparatus or process or both, provided herein is able to increase holographic image quality. Therefore, the claimed invention is directed to each incremental improvement alone or to any combination with other incremental improvements in the ultrasonic holographic imaging process and apparatus.